For animals living in the wild, nature plays for keeps. A life spent battling predators, famine, disease and the elements may be an independent one, but it can also be a very short one. That, at least, is the case zoos and wildlife parks often make when they contend that protective captivity may be a boon for many animals, particularly species that are endangered or threatened. But when it comes to at least one big and beloved creature, a new study suggests that a zoo might be the least safe place in the world.

In a survey of 4,500 captive elephants worldwide, a team of researchers from the U.K., Canada and Kenya found that once you lock up the giant, space-loving beasts, their health suffers, their median life span plummets, and they quit breeding  the last things you would want for a creature you're ostensibly trying to help survive. "Whether or not it's valid to say zoos keep species alive depends on which species you're talking about," says animal-welfare scientist Georgia Mason of the University of Guelph in Ontario. "Many species do well. Elephants don't." (See TIME's top 10 animal stories of 2008.)

Of all the findings reported in the new paper, it's the life-span numbers that are the most shocking. Among African elephants, zoo-born females live a median of 16.9 years in zoos, while those in the wild make it to a wizened 56. Asian elephants, the more endangered of the two species, live 18.9 years in captivity and 41.7 in the wild. A few superannuated wild elephants have actually reached their 70s, and in Kenya, from 30% to 50% of the noncaptive population hits at least 50. "So far," says Mason, one of the authors of the new study, published in the journal Science, "we've got 300 African elephants in zoos in Europe, and no one's yet reached 50."

Worse, what's killing the elephants is often ills they would never encounter in the wild. Obesity, for one: cage any healthy animal, feed it well and forbid it to move around too much, and it's likely to get fat. Cardiovascular disease is commonly reported among elephants, which, as in humans, can be a direct result of too many calories and too little exercise. What's more, baby elephants born in captivity are noticeably chubbier from the start than those born in the wild. That may be a result of the mothers weighing too much, but whatever the reason, Mason worries that as in humans, overweight juveniles are overwhelmingly likelier to grow into overweight adults, with all the attendant health risks.

Herpes, improbably, is killing elephants too  at least the Asian species. Wild African elephants are often infected with a form of herpes virus that causes them little illness or discomfort, but when the two species were brought together in zoos, the virus jumped to the Asians and mutated into a lethal form. "Zoos have accidentally created this," says Mason. "It's killing Asian elephant adults, and it's a leading cause of the species' infant mortality." (See pictures of Asian elephants.)

Another reason the babies are dying is, tragically, the mothers. Infanticide is almost unheard of among wild elephants. Mothers invest two years in their pregnancies, they live in stable matriarchal groups, and females collectively care for the young. In captivity, mothers are often held in relative solitude, undergo stressful and painful births, and then simply kill the source of all that suffering. Some mothers, Mason says, may even turn to infanticide because they just don't know what the small, squirmy creature that suddenly appeared in front of them is. "Many females in zoos have never seen a calf," she says, "so they may not recognize it."

Zookeepers and policymakers who aren't moved by all this suffering might instead be convinced by the simple fact that it costs a fortune to keep elephants so miserable. In the past 10 years, zoos have spent or committed to spend about $500 million to build or upgrade enclosures designed to improve the lives of 250 animals  but nothing so far suggests that does much to improve captive elephants' health or longevity. In Kenya, on the other hand, the wildlife service has an annual budget of just $20 million to look after tens of thousands of elephants. What's more, while Asian elephants remain in jeopardy  with only about 60,000 of them left  cost-effective wildlife-protection programs have allowed the African elephant population to rebound to a robust 500,000. "African elephants are a conservation success story," says Mason. That's true enough of much of the free population; not so much for the detainees.